In the conventional key switch of the kind mainly used in typing keyboards, the switching action is more responsive the closer the key is loaded at the center, so that it will be above the plunger guide. If there is such actuation on the key center, that is above the plunger guide, no tipping moments will be applied to the guide. In conventional keys, the size of the edges are substantially 1:1 forming a substantially square profile, and for a size of about 18.times.18 mm, no bothersome tipping moments arise. Furthermore, the finger-trough provided at the top of the key favors central actuation of conventional keys.
However, entirely different behavior upon actuation takes place with so called multiple-key units which are longer in one direction than the other, with the edge ratio on these multiple-key units being larger than 1:1. These multiple-key units are preferred in special operations, for instance, for the new line, shift or space keys. Because the key is longer in one direction, it is no longer centrally actuated, rather, as a rule, it is actuated eccentrically thereby causing tipping moments to the plunger guide.
When such a multiple-key key is depressed at its edge, and mechanical means are not provided below its top, then a higher force of actuation is demanded due to tipping moments caused by the eccentric loading which produces higher friction in the plunger guide. In extreme cases, key operation is no longer reliable, and the key may be pressed out of its mounting at the keyboard.
To overcome such difficulties, it is known to provide multiple-key units with an actuation mechanism consisting of a U-shaped wire connected in movable fashion, directly or indirectly, with the top of the base of the key. The connection locations at the top of the key are located near the outer key edges, with the free ends of the U-shaped wire passing through bore-holes in which they are held in both a tippable and displaceable manner. The bottom section of the wire is rotatably supported at the key base.
When eccentric actuation from the top of the key takes place, force is deflected by the wire which is rotatably supported in the key base and transmitted to the opposite edge of the key top. Thereby, even if the key is actuated eccentrically, the opposite side of the key top is also pulled down by means of a wire, whereby uniform force transmission is achieved and the guide means of the key plunger is free of any significant tipping moments. Thereby problem-free actuation shall always be obtained.
Even though such a supporting mechanism satisfactorily meets its required function, this known solution nevertheless has a serious drawback if the switch assembly and especially the key top are mounted automatically on the key base.
That is because the known U-shaped wire when processed on an automated machine can be threaded only with extreme difficulty into the support mechanism provided at the top of the key, and accordinly heretofore manual assembly has been preferred for the reason of cost and in light of the significant amount of rejects inevitable in the partly automated manufacture. Moreover, such a wire held at the top of the key does not lend itself to being mounted by mechanically pressing the multiple keys onto the key base, and again the need for manual assembly is necessary.
The object of the present invention is to overcome the drawbacks of the prior U-shaped wire, and this object is to change the support mechanism of the tops of multiple-key units so that they may be assembled automatically together with the single keys in one operational step. This drawback is solved in that the free ends of the U-shaped wire are bent inwardly approximately at right angles, and parallel to its crossed part with the cross-part of the wire being rotatably supported in mechanical supports in an eccentric manner at the top of the key and parallel to the longitudinal axis. The U-shaped wire is held in an obliquely assembled position by means of a stop, and the key base includes a wire guide means that can vertically receive the free ends of the U-shaped wire during assembly, with the free ends being horizontally displaceable when the key is actuated.
This design of the support mechanism allows automatic assembly of all key tops, i.e., single key units and multiple-key units, by applying pressure from above to the premounted units.